Solid–Liquid Equilibria of Sodium–Potassium–Cesium Sulfate System at 298.2 and 323.2 K

Abstract—The salt lake brine resources in the Qaidam Basin are characterized by abundant alkaline (earth) metal components, demonstrating significant developmental potential. To elucidate the crystallization behavior of salts in the sodium-potassium-cesium coexisting sulfate system, the phase equilibria of the quaternary system Na⁺, K⁺, Cs⁺//\({\text{SO}}_{4}^{{2 - }}\)–H₂O were systematically investigated at 298.2 and 323.2 K using the isothermal solubility. experimental measurements included the density, refractive index, and liquid-phase composition of the equilibrated system, while the equilibrium solid phases were identified by X-ray powder diffraction analysis. Key findings reveal that this quaternary system is a complex system. At 298.2 K, the phase diagram comprises four invariant points, nine univariant curves, and six crystallization phase regions, yielding a solid solution [(K₂SO₄)_x(Cs₂SO₄)\(_{{1-x}}\)] and double salt (Na₂SO₄⋅3K₂SO₄). In contrast, the phase diagram at 323.2 K simplifies to three invariant points, seven univariant curves, and five crystallization phase regions. Notably, Na₂SO₄⋅10H₂O was completely converted to Na₂SO₄ at 323.2 K. Comparative analysis demonstrates that the crystallization phase region of the solid solution [(K₂SO₄)_x(Cs₂SO₄)\(_{{1-x}}\)] remains predominant across both temperatures, indicating its preferential crystallization tendency with in this system.